Characterizing Thin-Film Stress Fields by Resonance of Membrane Arrays
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Characterizing Thin-Film Stress Fields by Resonance of Membrane Arrays R. Engelstad,a E. Lovell,a A. Chalekian,a S. Janowski,a M. Cash,a and H. Eguchib Computational Mechanics Center, University of Wisconsin-Madison 1513 University Ave., Madison, WI 53706 U.S.A. b TOPPAN Printing Co., Ltd., 4-2-3 Takanodaiminami, Sugito-Machi, Saitama, Japan 345-8508
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ABSTRACT Controlling thin-film stress magnitudes and nonuniformities is a persistent and pervasive challenge for applications ranging from the semiconductor industry to nanotechnology. Consequently, the ability to accurately measure the intensity and spatial distribution of film stress is essential for fabrication process optimization. This paper describes novel extensions of the membrane resonance method to characterize in-plane film stress gradients. Arrays of freestanding membrane windows are fabricated by first depositing thin films over a silicon substrate. Back-etching then creates an array of membrane windows which are sequentially resonated with a compatible drive electrode. The technique was used to determine stress uniformity across electron projection lithography masks, which incorporate membrane window arrays. In addition, a procedure is presented for identifying the two principal stresses in individual membranes. INTRODUCTION Induced substrate curvature is often used to estimate film stress, but is inaccurate except for uniform distributions. Alternatively, average stress in a freestanding membrane can be accurately determined from measurements of its fundamental frequency of vibration [1]. Resonance of membrane arrays with compatible drive electrodes facilitates the mapping of gradients in film stress over substrates [2]. In this work, the process was automated and carried out in a dedicated vacuum chamber at the University of Wisconsin Computational Mechanics Center (UW-CMC). A vacuum environment was required to eliminate deleterious squeeze-film and added-mass effects of the surrounding air. A scanning laser vibrometer was used to accurately identify natural frequencies and mode shapes. Frequencies are converted into membrane stress, window by window. In this paper, the procedure is demonstrated by generating film stress maps for an electron projection lithography (EPL) mask fabricated by TOPPAN Printing Co. (A typical 200-mm EPL mask has over 8,500 membranes.) EXPERIMENTAL DETAILS Figure 1 is a schematic of the large-format vacuum chamber constructed for membrane resonance testing at the UW-CMC. Within the chamber, the EPL mask is supported directly above a driving electrode that is used for excitation (as shown in Fig. 2). A scanning laser Doppler vibrometer is used to detect the out-of-plane response of the membrane as a range of frequencies is scanned. Mapping the surface of the freestanding
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film allows for the determination of the mode shape (as well as the natural frequency) to be used in the calculation of the membrane prestress.
Scanning Vibrometer Optical Mirror Optical Mirror
Diagnostic EPL Diagnostic Mask Mask Mask
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